To erase an image using morph; at the point in the timeline that you would like an image to disappear add another image, it can be anything. This image does not need to be next to the image you would like to disappear in the timeline but it must be after that image. Change the solidity of the new image to 0% meaning it is invisible. Set the invisible image to Morph and choose the image you would like to disappear as the' Morph from' image. The image will be morphed into an invisible image and will disappear.

In this example I have used the round image set to 0% solid and to morph from the arrow. I have set the Animate time to 0 seconds and this means the arrow just disappears.